Make objects_path not required

[rm--]

Some JSON responses are a list, so objects_path must be set to an empty tuple.

So it's easier for the user if objects_path would be an empty tuple by default.

[rm--]

In MusicbrainzResource objects_path must still be set to ('',). A user of durga shouldn't have to do that.
If objects_path isn't set at the resource and a schema is set, validation fails.

[keimlink]

Thanks for reporting the issue. Could you please create a pull request with a test (and a fix if possible)?

It's fine if you can reuse the MusicBrainz resource for the test, so you don't have to create a new one.

[rm--]

There is a test, a fix and a corrected test. Watch out the commit messages for explanation.